In a wheel loader or other automatic work vehicle, traction force (travel drive force) and work implement drive force are obtained from an engine as a shared drive source. For example, traction force is obtained from a so-called HST hydraulic travel device or via a torque converter. The work implement drive force is obtained via hydraulic pump driven by the engine.
In such a work vehicle, there are many cases in general practice in which travel, loading, and other work are carried out simultaneously, and it is important to distribute the output of the engine with good balance to travel and to the work implement. For example, in excavation, the vehicle is made to move forward to thereby press the work implement into a mound of soil, and the work implement 3 is raised. Soil is thereby scooped into the work implement. Therefore, it is important to balance the traction force for causing the vehicle to travel and the lift force as a work implement drive force for raising the work implement (see FIG. 14(a)). However, skill is required in order to operate the vehicle so as to capably obtain this balance. For example, when an inexperienced operator excessively depresses the accelerator and presses the work implement into the soil during excavation, the vehicle cannot move forward and comes to a stop. In this state, the traction force of the vehicle is excessively high and the lift force is reduced (see FIG. 14(b)). For this reason, the work implement cannot rise even when the work implement operation member is maximally operated. In such a state, the hydraulic circuit for feeding hydraulic fluid from the hydraulic pump to the work implement enters a relief state in order to protect the hydraulic pump. In this stalled state in which the vehicle has stopped moving, the engine output continues to be high and fuel consumption (amount of consumed fuel) is therefore increased.
In view of the above, a technique for suppressing the increase in fuel consumption has been proposed for when the vehicle is in a stalled state as described above. For example, the actuation speed of the work implement is detected in the technique disclosed in Japanese Laid-open Patent Application No. 5-106243. The engine speed is reduced until the actuation speed reaches a predetermined value when the vehicle is in a stalled state.